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Scott, I'd certainly query the air turbulence idea for the cause of vibrations at particular frequencies/revs; turbulent flows create broadband noise & would reduce the chance of modal vibration; I'd not expect the air itself to be the driving force of the vibrations, more likely transmitting through the metal itself. Check welds, if they're starting to rot, they can make a hell of a racket; rotten, loose heatshield as above is another possibility, but if you've knocked out the inner cat meshes via the outer pipes then it's unlikely you've got it all out. If you've access to a welder, could you cut a panel out of the top side of the middle section of the cat, clear all the mesh out & then weld it back together? Or if not a welder, use some metal strip (large tin cans), jubilee clips or a riveter & a load of gun gum. It might sound rather 'scrapheap challenge' but I've put cars & bikes through MOTs with exhausts patched with tin cans. Actually, my bike went through its last MOT with a combination of whiskers & john smiths, although I enjoyed emptying the latter more Power steering fluid - stick to genuine honda. It's not that pricey, you don't need loads & while other fluids may be fine, mixing different fluids can cause issues.

Oh wow that is close, nobody expected that !!!!!

yes mate ha ha its gone now dont worry!!